1. Field of the Invention
The invention relates to the field of removal of material from wafers and more particularly to free hold down systems for wafers during such processing.
2. Prior Art
The prior art uses solidified wax to retain the wafers to be lapped or polished in position on a wafer mounting plate. Following completion of the lapping or polishing, the wafers are removed from the mounting plate by chilling the mounting plate, the wafers and the wax to render the wax brittle and then applying a small mechanical shock or force to the wafer to fracture the wax and release the wafer from the mounting plate. The wafers are then cleaned in a sequence of solutions to remove residual wax and other materials which are detrimental to the intended use of the wafer. This process is relatively expensive, first because of the cost of the wax, second because of excessive breakage of wafers during the removal and cleaning processes and third because of the cost of the removal and cleanup steps.
Because of the drawbacks of the waxed-down wafer mounting process, it has been an objective in the wafer processing industries to develop the ability to free mount wafers without the use of adhesive materials.
U.S. Pat. Nos. 3,841,031 and 3,857,123, both to Walsh, disclose a free polishing system for wafers using single-wafer chucks and dynamic loading of the chucks. These patents disclose the use of a poromeric material as the wafer mounting surface of a wafer chuck. A material such as that sold by Dupont under the tradename "Corfam" is used. The smooth or front surface of the poromeric material contacts the wafer. The wafers are mounted by moistening the pad with water and placing the wafers on the pad. No particular wafer characteristics or wafer preparation procedures are specified in these patents as being required for proper operation of the system. These patents assert that their process is useful for all types of wafers, including semiconductor materials, refractory oxides and magnetic bubble materials. So far as the present inventors are aware, the technique of these patents has not become a commercial success.
U.S. Pat. No. 4,132,037 to Bonora discloses a method of adhering a plurality of wafers to be polished to a polymeric pad mounted on a wafer mounting plate. The adhesion is obtained in a liquid free manner via direct solid-to-solid wetting characteristics at the exposed surface of the pad with the material of the wafer. Like Walsh, Bonora specifies no particular wafer characteristics or wafer preparation procedures for use on the wafers prior to mounting on the pad. Bonora teaches use of a heat press station at 125.degree. F. (52.degree. C) for a period of one minute at a pressure of about 1 PSI in order to seat the wafers. In order to remove the wafers from the mounting pad, Bonora teaches cooling the wafers and mounting material to 2.degree. C. after which a thin bladed tool is inserted under each wafer to break the bond. Bonora indicates that his technique is suitable for wafers 15-25 mils thick.
A simplified, room temperature free mounting technique is needed which is effective for multiple wafers mounted on a single wafer mounting plate and with wafers less than 15 mils thick.